The present invention relates to a method and apparatus for heating process gases for industrial processes to a particular desired temperature. Process gas, particularly heated air, is used in considerable quantities in many industrial processes. Conventional methods of heating large volumes of gas, for example by means of heatexchangers, require considerable capital investment.
Recently, therefore, use is more and more frequently being made of the combustion of fossil fuels such as coal, coke, natural gas, oil, etc. Both from the environmental and the technical points of view, such combustion causes problems. From the environmental point of view, the discharge of sulphurous compounds results in the acidification of the environment and deposits of smoke and soot. Technical problems arise since sulphur may not be present in certain processes, such as various iron and steel manufacturing processes. Added to this are adverse cost factors since the prices of fossil fuel continue to rise substantially.
Various solutions to the above problems have been proposed. One method has been developed in the steel manufacturing field for increasing the temperature in the blast gas in a blast furnace, with the object of increasing production and at the same time reducing the coke consumption. In this known method, the blast gas is passed completely or partially through a plasma generated by means of an electric arc in a plasma generator of known type. The advantages with a plasma generator are its high degree of efficiency, reaching almost 90%, and that an extremely high temperature can be achieved, usually in excess of 3000.degree. C.
In a plasma gas generated by a plasma generator, some of the atoms and molecules are ionized and these ionized particles are extremely reactive. When a plasma gas produced from an air-flow reverts to normal conditions at lower temperature, however, nitrogen oxides are obtained as well as nitrogen and oxygen. Nitrogen oxides are extremely toxic and give rise to the formation of nitric acid which can destroy the process equipment.
However, ih the above known method of heating the air blast for blast furnaces, the formation of nitrogen oxide has not been taken into consideration since the plasma gas generated is blown directly into the blast furnace where the nitrogen oxides are automatically broken down during their passage through the blast furnace charge. The use of a plasma generator for heating process gases is thus heretofore substantially limited to blast furnace applications.